Described below is a method of bandwidth allocation in VoIP telecommunication systems in which bandwidth is allocated according to dynamic rate based techniques.
The method is explained in its possible uses in satellite telecommunication systems.
In satellite communications, bandwidth is scarce and therefore expensive. Scarcity of satellite bandwidth requires the reutilization of bandwidth by suitable centralized methods of access controlled by a hub-gateway device. Such centralized methods of bandwidth rate control has the major drawback of implying round trip satellite link delays for bandwidth allocation to remote stations to be accomplished by the hub-gateway, as it is schematically illustrated by FIGS. 1 to 5.
FIG. 1 is a block diagram schematically illustrating an example architecture of a satellite communication system, partially packet-based, as known from the related art.
A communications satellite SAT is in communication with a plurality of remote satellite stations VSAT, also called Very Small Aperture Terminals (VSATs), and with a hub-gateway HG.
An example of IP broadband satellite system is Digital Video Broadcasting-Return Channel Satellite system (DVB-RCS), as defined by ETSI-standard EN 301790 [1]. The remote satellite stations VSAT, in DVB-RCS terminology, are called Return Channel Satellite Terminals(RCSTs).
Other examples of IP broadband satellite systems include ViaSat Docsis and Hughes IP-OS.
The hub-gateway HB is a device responsible of controlling bandwidth assignment upon requests arriving from the remote satellite stations VSAT. The hub-gateway HB is, on one side, in communication with the remote satellite stations VSAT, via the geostationary communications satellite SAT, on the other side with one of more networks, e.g. ATM, IP, PSTN, GSM.
Each remote satellite station VSAT is in communication, via an Ethernet interface EI, with an access device AD1, AD2, AD3, directly, as for examples in case of VoIP gateway AD1, or, indirectly, as in case of a presence of a Ethernet switch ES between the access devices CPG, SIP-phone, BTS AD2, AD3, AD4 and the remote satellite terminal VSAT.
Examples of access devices AD1, . . . , AD6 include H.248/SIP/MEGACO access gateways, CPGs, mini trunking gateways, base stations, NANO BTSs, SIP phones.
Device A.BIS denotes the interface between the base transceiver station AD5 and the base station controller, not shown.
There are several known schemes for requesting bandwidth capacity by remote satellite devices VSAT.
A first known scheme for requesting bandwidth capacity is a scheme in which dedicated bandwidth is allocated between the remote satellite station VSAT and the hub-gateway HG based on some signaling from the access devices AD1, . . . , AD5 or as a result of a configuration action taken by the hub-gateway HG itself. With dedicated bandwidth allocation schemes, the operator typically allocates to a remote satellite terminal VSAT a fixed bandwidth, e.g. 20 kbps, which is always there even if no data is being transmitted over the Ethernet interface EI.
A second known scheme is for requesting bandwidth capacity is a scheme on which bandwidth is allocated dynamically based on the packet rate. The allocated bandwidth depends on the packet rate arriving at the remote station VSAT from its connected access device AD1, . . . , AD5.
In DVB-RCS systems, examples of the above mentioned two known schemes for bandwidth allocation are Rate-Based Dynamic Capacity (RBDC) and Continuous Rate Assignment (CRA) corresponding to rate-based bandwidth allocation and to dedicated bandwidth allocation, respectively.
RBDC allocation has the advantage that it avoids the bandwidth waste of CRA dedicated allocation for allowing bandwidth reutilization. In RBDC allocation, the remote terminal VSAT keeps asking the hub-gateway HG for bandwidth as VoIP packets flow from the VoIP access gateway AD1, . . . , AD5 to the remote terminal VSAT and it ceases to do so as soon as packets stop being sent.
FIG. 2 is block diagram schematically illustrating an example of a rate-based dynamic bandwidth allocation during the set-up phase of a VoIP call as known from the related art.
Before the VoIP call connection is set up, the access gateway device AD is in a sleep mode, i.e. there is no activity in the communication with the remote satellite terminal VSAT, since no traffic is present. At a certain moment in time, when a subscriber goes off-hook, the access device AD itself issues a dial-tone, the subscriber starts to dial, the access device AD bitmaps the incoming dialing digits and when the subscriber ends dialing, signaling packets are sent to the remote satellite terminal VSAT.
In step ST21, packets P1, . . . , P4 start to flow from the access device AD to the remote satellite terminal VSAT.
The remote satellite terminal VSAT has to send these packets P1, . . . , P4 to the hub-gateway HG over the satellite link, which is the communications path between the remote satellite station VSAT and the hub-gateway HG via the communications satellite SAT. However, since before, no traffic was going through, due to the rate-based dynamic bandwidth allocation, no bandwidth was previously allocated for this task. Thus, in step ST22, the remote satellite terminal VSAT requests bandwidth allocation to the hub-gateway HG on a RBDC basis.
Thus, with RBDC allocation, the packets P1, . . . , P4 sent over the Ethernet interface EI from the access device AD to the remote terminal VSAT make the remote satellite terminal VSAT react and ask, in step ST22, to the hub-gateway HG for bandwidth equivalent to the rate being transmitted.
However, the bandwidth request of step ST22 to the hub-gateway HG implies a satellite round-trip delay between the remote satellite terminal VSAT and the hub-gateway HG.
In fact, the bandwidth request of step ST22 arrives to the hub-gateway HG via the satellite link and, in turn, in step ST23, the hub-gateway HG has to communicate to the remote satellite terminal VSAT when the bandwidth is actually allocated. Only then, the remote satellite terminal VSAT is able to transmit, in step ST24, according to the allocated bandwidth.
The round trip delay is often perceived by subscribers. In fact, the round trip delay may be of the order of ca 500 ms if propagation time alone is taken into account but other factors may contribute to longer delays. Thus, with RDBC allocation a first drawback is that, at the start-up phase of a VoIP phase, the packets P1, . . . , P4 arriving at the remote satellite station VSAT sit on an input buffer, not shown, in the remote satellite terminal VSAT, waiting for system reaction and for bandwidth allocation, thus causing a jitter at the beginning of the VoIP communication.
In FIGS. 2 to 7, the connections among the remote satellite terminals VSAT, the communications satellite SAT and the hub-gateway HG, denoted with a thick black line are connections in which data is transmitted, as for example in steps ST24, ST34, ST64, while the connection denoted with a dashed line are connections in which internal control information between the hub-gateway HG and the remote satellite VSAT is transmitted as for example in steps ST22, ST23, ST32, ST33.
FIG. 3 is a block diagram schematically illustrating an example of a rate-based dynamic bandwidth allocation with silence-suppression as known from the related art.
FIGS. 4 and 5 are the continuation of FIGS. 3 and 4, respectively.
In FIG. 3, at the set up of the VoIP call, in step ST31, the packets P1, P2 produce a bandwidth request. Operations performed for packets P1, P2 in steps ST32, ST33, ST34 are the same as the operations performed in steps ST22, ST23, ST24 of FIG. 2 for packets P1, . . . , P4.
Step ST31a represents a period of silence in voice communication which is occurring between packets P1, P2 and packets P3, P4.
With RBDC allocation in which voice activity detection/silence suppression is activated, when the subscriber of the established VoIP call goes silent, since there are no packet transmitted over the Ethernet interface EI for a while between the access device AD to the remote satellite terminal VSAT, the rate ceases to be there and therefore the remote terminal VSAT does not keep asking the hub-gateway HG over the satellite link for bandwidth, and the hub-gateway HG smoothly knocks down the previously allocated RBDC bandwidth for the VoIP call.
As shown in FIG. 4, in step ST41a, there is a period of silence in which packets are not sent until, in step ST41, new voice packets P3, P4 are sent. Due to the silence period of step ST41a, bandwidth is de-allocated in step ST42. As a consequence, when the subscriber starts talking again, the new voice packets P3, P4 have to sit for a while on the input buffer, not shown, of the remote satellite terminal VSAT until the satellite system reacts and allocates RBDC-based bandwidth again, as shown in FIG. 5.
Unfortunately the fact that the voice packets P3, P4 have to sit in the input buffer creates jitter during VoIP communication.
Hence, as above explained, the major drawback of known rate-based dynamic bandwidth allocation methods is that, in VoIP communication over satellite system, jitter is caused at the beginning of a VoIP call connection and, during a VoIP call connection, in case silence suppression is activated.